This application claims the priority of Germany Patent Document 198 57 785.0, filed Jun. 15, 1998 and European PCT/EP99/08392 file Nov. 3, 1998, the disclosures of which are expressly incorporated by reference herein.
The invention relates to a method for forming the mixture in a combustion space of an internal combustion engine, in which fuel is injected into the combustion space with a varying fuel mass flow during an operational cycle of the internal combustion engine.
A method of forming an ignitable fuel/air mixture in the cylinders of a direct-injection internal combustion engine is disclosed in German patent document DE 196 42 953 C1. In this method, fuel is injected into each combustion space by means of an injection valve with a variably adjustable opening lift. The valve lift is adjusted as a function of the operating condition of the engine; in particular as a function of the engine speed, the induction mass flow, supercharging, emissions and/or the smooth running of the internal combustion engine. Provision is also made for varying the opening lift of the injection during the injection procedure and, in fact, either by means of a brief increase in the opening lift in a central region of the injection period or by means of a dynamic reduction in the opening lift from a large initial value at the beginning of the fuel injection to a reduced final value, at the end of the fuel injection. The dynamic reduction in the opening lift takes place in accordance with an inverse exponential function, the major proportion of the metered fuel quantity being injected at a high fuel mass flow at the beginning of the injection period into the combustion space.
In method of operating an internal combustion engine disclosed in German patent document DE 196 02 065 A1, a main fuel quantity is first injected into a piston combustion space during a compression stroke phase and, after an injection pause of a duration which can be fixed, an ignition fuel quantity is injected into the piston combustion space. This achieves layering of the charge of injected fuel, the two mixture clouds overlapping or bounding one another at the time of ignition.
In internal combustion engine known from German patent document DE 195 30 072 A1, engine fuel is injected into a combustion space in two partial quantities, the first (and larger) mixture quantity being separated from the second (smaller mixture) quantity at the time of ignition. Injection of the first fuel partial quantity takes place during the induction stroke whereas the second fuel partial quantity is injected and ignited in a separate combustion space section in the expansion phase.
One object of the present invention is to provide a method for forming a mixture in a combustion space of an internal combustion engine, which method achieves an improved operating behaviour of the internal combustion engine, by special configuration of the injection procedure.
This and other objects and advantages are achieved by the method according to the invention, which provides for a three-stage fuel injection in which an additional fuel quantity is interposed between a main fuel quantity and an ignition fuel quantity. Due to injection of the additional fuel quantity between the main fuel injection and the ignition fuel injection, the time for the beginning and ending of the injections of the main fuel quantity and the ignition fuel quantity can be selected in a more flexible manner. A small fuel quantity at a relatively low fuel mass flow (i.e., with a small opening of the injection valve), is preferably injected into the combustion space as the additional fuel quantity. The main fuel quantity and ignition fuel quantity are connected in such a way that reliable ignition of the main fuel quantity takes place.
In one embodiment of the invention, the main flow quantity and the ignition fuel quantity are injected into the combustion space at a fuel mass flow which is large and essentially of equal magnitude (i.e., the same opening position of the injection valve). In this arrangement, an injection valve control system drives the injection valve into only two different opening positions. The method can therefore be effected in a particularly simple manner.
Another embodiment of the invention provides for a fuel injection whose fuel mass flow is lowered steadily from a (non zero) maximum initial value down to zero. In this arrangement, the injection valve is closed slowly, either continuously or in steps, and a variation of the air/fuel ratio in the mixture cloud from rich to weak is achieved. This technique ensures that an ignitable mixture with a favourable fuel distribution is present at some position of the mixture cloud at a certain time.
In a further embodiment of the invention, the injection valve is transferred from an open position into a closed position continuously, or in steps of preferably equal magnitude, so that the fuel mass flow correspondingly decreasing to zero.
Finally, in yet another embodiment, fuel is injected as a function of the mass flow of the combustion air flowing into the combustion space. In this arrangement, the fuel mass flow is, at least at times, proportional to the air mass flow so that an approximately constant air/fuel ratio is achieved in a region of the combustion space. For this purpose, the needle lift of the injection valve, in particular, is matched to the movement of the air inlet valve.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.